Airway traffic control system



J. 4i, @4%. o. s. FIELD Erm,

AIRWAY TRAFFIC CONTROL SYSTm 9 Sheets-Sheet 2 Fvled July 29, 1.944

The/'f Gltorncg Jan. 4, i949.

o. s. FIELD Erm.)

AIRWAY TRAFFIC CONTROL SYSTEM 9 Shseitsrshgt 3 Buventors @f/d @d w FiledJuly 29. 1944 Jan, 4? E949.' o. s. HELD Wm.

Filed July 29, 1944 AIRYJAY TRAFFIC CONTROL SYSTEM 9 Smets-Sheet 4Snnentors FMC Jan. 4, 1949. o. s. FIELD Erm.

AIRWAY TRAFFIC CONTROL SYSTEM Y Filed July 29, 1944 9 shuts-Shut 5 EnAc,f

Snuentors ama/d mdf/www# Jan. 4, 1949. o s. FIELD ETAL AIRWAY TRAFFICCONTROL SYSTEM mf u M a w MW w m l .M u 8 f. m M 9 .a m ,m f m. @L Emmath M w v. HW NOWMMN .w N NNUU n Nl- F Jan. 4, i949. o. s. FlELD ETALAIRWAY TRAFFIC CONTROL SYSTEM 9 Sheets-Sheet 7 Filed July 29, 1944.

Fsc. DXE

w-Ea

ALTITUDE-X2 jan. 4, 1949. 0, s, HELD ETAL 2,458,361

AIRWAY TRAFFIC CONTROL SYSTEM Filed July 29,1944 9 Shets-$heet 8Snventors Jan. 49 E949. o. s. HELD ETAL 2,458,361

AIRWAY TRAFFIC CONTROL SYSTEM Filed Julgyv 29, 1944 9 Sheets-Sheet '9:inventors Patented `Fan. 4, 1949 AIRWAY TRAFFIC CONTROL SYSTEM Oscar S.Field and Sedgwick N, Wight, Rochester, N. Y., assignors to GeneralRailway Signal Company, Rochester, N. Y.

Application July 29, 1944, serial No. 547,175

(ci. 34a- 112) 53 Claims. l

This invention 'relates to a combinedairway Ytrame route defining andairway traillc condition signalling system, and more particularly tosuch a. system in which airway trailic conditions in advance areindicated in the cab of an airplane automatically irrespective ofweather and fog conditions. This invention is to be considered as .animprovement over the prior applications of Wight, Field and Saint, Ser.No. 517,814, led January 11, 1944, and Dicke, Ser. No. 532,181,

A led April 2l, 1944, which issued April 20, 1948 as Patent No.2,439,846, which disclose only airway route defining apparatuses.

A number of diflqcult and important problems present themselves inthe-dispatching of large numbers of airplanes over specific air routesirrespective of 't e presence of fog. and clouds, and more partire arlywhen vision is very poor due to such atmospheric condition. 'I'he firstof these problems is due to the possibility of having too many airplanesarrive at their destination airport at the same time and the solution ofthis yproblem resides in the provision of means for starting out theseairplanes at their starting points at the proper times to prevent suchcongestion, and the second problem, namely, the problem of the presentapplication, is not concerned with this rst problem in that it islimited to apparatus'for governing the progress of airplanes overparticular routes and to the crossing of other routes by such airplanes.

This second problem resides in the proper spacing of airplanes dying ina particular direction over a route irrespective of weather and fogconditlons and in providing proper means whereby airplanes ying on tworoutes crossing each other will not cross at the same altitude at thesame time, i

In accordance with the present invention it is proposed to employ radiocommunicating facilities to distinctively manifest at successive xes orstations along the route the presence of airplanes in the variousaltitude layers over such route and between that x and the next ilx inadvance, and in the provision of radio communieating means forretransmitting such information of occupancy from such nx to approachingairplanes in the rear.

One of the objects of the present invention is to provideach waysidestation or fix with a singlev radio Yreceiving antenna and a singleradio transmitting antenna, both of which are preferably directional,and which may receive and transmit, respectively, radio energy which isdistinctive of tra'ic conditions in advance for each 2 of the variousaltitudes and in the provision of airplane carried radio receiving meanswhich normally responds only to'radio energy characteristic of airwayktraic in advance of the airplane in the particular altitude at whichsuch airplane is flying.

Another object of the present invention resides in the provision of ascanning radio receiving antenna and an associated kinescope whichpictorially indicates to the pilot irrespective of weather or fogconditions the locations of one or more ground radio transmittingstations in advance of the airplane on the route. It should beunderstood that normally these pictorially indicated radio transmittingantennae would emit coded energy which would cause the spot 0n thekinescope identifying such ground radio transmitting station to ash incode to not only indicate traillc conditions in advance for thataltitude but which will also be coded or otherwise characterized toidentify the particular way station at ywhich such transmitting antennais located.

Another object of the present invention resides in the provision ofmanually operable selecting means whereby the radio receiving apparatusincluding the scanning antenna and kinescope above mentioned, may renderthis receiving apparatus-no longer responsive to the radio energycharacteristic of the altitude at which the airplane is lying but willreceive energy assigned only to some other altitude as for instance thenext adjacent altitude above or the next adjacent altitude below thealtitude at which such airplane is flying, so `that the pilot may atwill by manual operation of such manually operable selecting means'observe trame conditions in such other and adjacent altitude. It is ofcourse understood that such looking, so to speak, into'another altitudewould ordinarily be. confined to the next ,ad--

jacent altitude either above or below' the altitude at which theairplane is flying but the proposed apparatus is not restricted to suchlimited use. This feature of applicants invention enables a pilot toascend to the next/adjacent higher altitude or to descend to th nextadjacent lower altitude in the event the sectionedirectly vin advancePof him, at the altitude at whiclheisy-'f" ing, is occupied by anotherairplanehaving a considerably lower cruising speedthan the airplanewhich he is piloting.

Another object of the present invention resides in the provision ofmeans on an airplane for transmitting a rearwardly directed radio beam Ito the ground stations or xes in the rear so as to manifest its presencein an airplane in the second section or the section next in the rear atthe same altitude. ThisV emitted radio beam will of course becharacteristic of the altitude at which the airplane is ilying and willcause the ground station next in the rear of the airplane to emit radioenergy characteristic of such tramc conditions and also characteristicof the altitude at which the airplane responsible for such transmittedenergy is flying.

Another object of the present invention resides in the provision ofmeans to characterize the radio energy rearwardly emitted from anairplane so as to identify that particular airplane, provision beingmade on the ground for repeating such airplane identifyingcharacteristic to the next ground station or fix in advance and to causethe enerby transmitted from such ground station next in advance back tothe same airplane whereit originated, so as to identify that airplane.The net result of this repeating of the4 airplane identifying code isthe airplane may observe and check the operativeness of its sendingapparatus and also the operativeness of the ground radio receivingapparatus in the rear and 'the ground radio transmitting apparatusdirectly in' advance of the airplane in ight. This checking provisionshould provide the necessary check against failure of apparatus reliedupon to indicate dangerous trailc conditions in advance of an airplane.

Another object of the present invention resides in the provision ofmeans for operating contact mechanism for manifesting the altitude atwhich an airplane is flying. This contact mechanism is to manifest suchaltitude in-conformity with the indication given by a rather delicatelydesigned altimeter. In accordance with this object of the presentinvention it is proposed to employ a light beam which swings through anarc in accordance with the swinging of an indicating point in analtlmeter and in the provision of means for operating contacts to aposition corresponding to the position assumed by such light beamthrough the medium of a selenium cell, photo-electric cell or othersuitable light responsive apparatus.

Another object of the present invention resides in the provision ofautomatically controlled interlocking means at a common fix or stationwhere two airway'routes cross each other at the same altitudes andincluding means for displaying a stop or hold indication for aparticular altitude on one route when there is an airplane approachingat that altitude on the other route, whereby the crossing point of thetwo routes for a particular altitude may be alloted to that airplanewhich may loop sdewise in the event the section inl advance is occupiedas a result of which he must hold, so to speak, to avoid ying into anoccupied section;

Fig. 3 illustrates conventionally' an airplane equipped with airplanecarrying apparatus embodying the present invention;

Figs, 4A-4C illustrate the ground located apparatus of an eastboundroute embodying the present invention;

Figs. 4DX1 and 4DX2 illustrate automatic interlocking apparatus for acommon nx at an airway crossing;

Figs. 5A, 5B and 5C illustrate an altimeter controlled contact mechanismconstituting part of the airplane carried mechanism illustrated in Fig.3; and

Fig. 6 illustrates a modified form of the ground located apparatus shownin Figs. 4A-4DX which operates to give 'a distinctive hold indicationunder certain conditions of failure of the approach control apparatus.

General discussion In order to obtain a better understanding of themanner in whichthe apparatus of the present invention is to be usedattention is directed to Figs 1 and 2'of the drawings.

In Fig. 1 has been illustrated a portion of an air-route including waystations or fixes A', B, C and DX for altitudes from 2000 feet to 9000feet,

A inclusive. In altitude 2 have been illustrated airplanes IP2, 2P2 and3PZ; in altitude 3 has been illustrated airplane IP3. These airplanesare also shown in Figs. 4A-4C. In altitude 4 have been illustratedairplanes IPI and 2P4; and in altitudes 5, 6, 7, 8 and 9 have beenillustrated airplanes IPS, IPG, IPI, IPB and-IPS respectively.

Although, as will be` more fully pointed out hereinafter, fix A, B, C orDX is each provided with only a single radiating or sendlngantenna andthis radiating antenna in accordance with the particular inventionillustratedV radiates only a single carrier frequency to airplanes inthe rear, this carrier frequency is preferably modulated at a pluralityof frequencies, one modulating frequency for each altitude, so that thepilot in a particular airplane will observe .on the screen S (Fig. 3) ofthe kinescope carried by his airplane traflic conditions for hisaltitude only for particular ground stations, whereas other airplanesflying at different altitudes will distinctively be informed as totrailic conditions in advance for their altitudes from the same emittedbeam which emits distinctive radio energy to airplanes for allaltitudes. It should, however, be understood that a separate carrierfrequency may be used for each altitude for each direction oftransmission to and from airplanes. By reason of the extremely highfrequency radio carrier frequency proposed to be used (ultra-highfrequency) the modulating frequencies may also be very high and would inpractice be high enough so as to be inaudible. This inaudibility isdesirable in order to be able to properly discern the code superimposedon the modulating frequency.

This characteristic featurel of applicants invention is clearlyexemplified by the various indications given forthe various altitudes 2to 9 for ilx B, for instance, in Fig. 1 of the dra-wings. It may bepointed out that the solid letters G and R signify proceed and holdtrale conditions in ad- Vance whereas dotted letters G and R whenaccompanied with the solid letter N signify that normal trafilcconditions are manifested for that altitudewhereas the associated dottedletter G (see fix B altitude 5000) will manifest that the trafilccondition for this -altitude and fix is potentially proceed. Thisproceeduindication is not normally manifests normal traii'ic conditions.The pilot of A an airplane flying that altitude but to the rear of a xwill of course observe an R signal only (not the N) due to approachcontrol that his airplane exercises, whereas if there is no suchapproaching 5 airplane in that altitude and a'pilot from an airplane inanother altitude sets his hand knob HK (Fig. 3) to look for traicconditions in the first mentioned altitude will observe traiilccondition N. For instance, the tramo manif tation for altitude 8, fix C,is N followed by a dotted letter G which signifies that trafdc conditiare potentially clear but that the airplane IPS, which has exercisedapproach control over fixes A and B, has not yet exercised approachcontrol over fix C. Indication G for altitude-'hat ilx C indicates thatthe approach control imposed by airplane IPi extends far enough to havechanged the trac I manifestation from normal (N)=to.proceed(G)/.""`

in the same section he is in but one altitude higher, an unsafecondition. In this connection'it may be pointed out that a. pilot wheninvestigating trame conditions in adjacent altitudes must observe -atramo aspect of N/N in order to be permitted to fly into such adjacentaltitude. As -aspect N/N in an yadjacent altitude manifests to the pilotthat the block to the rear. the block in which he is ying, and the blockin advance in such adjacent altitude is unoccupied, which is a safecondition for entering such adjacent altitude. This feature ofapplicants invention is due to the approach control provided and will bemore specifically described hereinafter.

It may be pointed out at this time that there is a possibility of apilot, after observing favorable trafiic conditionsin an adjacentaltitude, will en- #Ater such altitude and will then flnd that traicconditions have changed in the meantime by rea- Similarly, the traiiicmanifestation for altitude 6.-. son of an airplane approaching from therear in x B, is N followed byv a dotted letter R which signifies thattraillc condition for this nx altitude is potentially danger, meaninghold, and that no airplane is approaching from the rear at close enoughrange to exercise its-approach control over fix B.

Referring now to Fig. 2 of the drawings the dotted circle or loop 60illustrates the path over which the airplane IPZ may circle to eiiect aholding operation. That is, in accordance with the present invention anairplane is not allowed to pass a x manifesting danger, that ismanifesting hold traffic conditions, and if an airplane is approachingsuch nx he must hold by a circling operation until traiiic conditions inadvance clear up and manifest proceed. Similar circling loops 6I and 62have been illustrated for airplanes 2P! and 3PZ respectively. Airplane2P! is not required to hold but may hold on his own volition. The mannerin which a ground radio station maintains a danger signal aspect (R)while an airplane is circling in its holding circle in the block inadvance is more specifically pointed out in the operation of the systemhereinafter. A

Referring again to Fig. l of the drawings under normal conditions apilot of an airplane flying at a particular altitude will observe on thescreen of his kinescope the particular tralc conditions existing for hisaltitude at one or more fixes in advance of the airplane, all asillustrated by the solid line letters assigned to the fixes in advanceof the various airplanes, each for its particular altitude. It is,however, desired to point out that a pilot of an airplane flying at aparticular altitude, as for instance airplane IP3 flying to the rear offix A in altitude 3, may by operating the hand knob HK (Fig. 3) to the 4position (meaning altitude 4000) and have the aspect G/G indicated forfixes A -and B in Fig. l for altitude 3000 changed to an aspect R/G (Rover G, the last letter meaning the first signal ahead) indicatedforaititude 4000 for fixes A and B in Fig. 1. In other words, if thepilot vof an airplane nds trame conditions in advance in his altitudeunfavorable he may look, so to speak, into the adjacent altitude aboveor below the altitude in which he is flying to see whether trafficconditions in one or the other of these'adjacent altitudes is favorable,-and if this is the case he may change altitudes in order to hurdle aslower airplane in advance in the altitude "assigned to him. Inthe casejust considered such change of altitude can however not be'made becausethe pilot has been informed that approach control is in effect at bothof fix-altitudes A4 and Bd, meaning that there must be an airplaneflying 75 such adjacent altitude. In this event it will be necessary forhim to return to his original altitude. This is perfectly safe in thatthe holding feature of the system whichis-devised 'to allow a pilot tocircle to eiect a holding operationgwill hold trame conditions in hisoriginal altitude fr a time long enough for him to have returned fromsuch adjacent altitude to his original altitude.

Structure Airplane carried structure-Referring toFig. 3 offthe drawingsit will be observed than an airplane IP! hasbeen illustrated equippedwith suitable airplane carried radio communicating i-apparatus. Near thefront of the airplane and constituting part of this apparatus is ascanning antenna SA which may be of any suitable construction-but ispreferably of a construction such. as illustrated in the priorapplications of Wight, Field and Saint or Dicke, above referredt/Thisscanning antenna SA, illstrated conventionally as a scanningshaft'isupported at right angles on a scanning shaft, is cted throughthe mediumof a scanningwrecei er SAR, to suitable nltering apparatusFSI, and altimeter controlled cmg-amid a push button PB, to a kinescopeincluk creen S. By reference to the cepending application of Wight,Field and Saint or Dicke just referred tovit-wi1l bey readily understoodthat this screen S will have dispiayedthereon pictorially each of theactive ground radio transmitting stations in advance oi. the airplanewithin l radio communicating distance, and juxtaposed thereon insubstantially the same relation to each other as they are located in thefield. It 'should -belunderstood that theY-lterportions F2 and F3 of thefilter FSI are constructed toaadmit only received radio energycharacteristic of'A the altitude 2 or 3, as the casemay be, then seletedby the contactZ of the altitude contact mechanism AC more specificallyshown in Figs. 5A, 5B and 5C of the drawings.

This receiving apparatus also includes a hand knob HK which maybemanually operated by the pilot and by the use of which the pilot mayobobserve trame conditions in advance in other altitudes than theparticular altitude at which he is flying. It will be observed that thecontact 20 controlled by the altimeter contactor AC is shown in the`position 2 Vwhere it selects the filter portion F2 of the filter FSIwhich signifies that the airplane IP! is flying at altitude 2000 as aresultvof which there is indicated on the screen S trailic conditions inadvance of this airplane IP2 for altitude 2000. If now the pilotoperates his hand knob HK to the 3 position. signifying an altitude of3000 feet. and if he then depresses the push button PB the screen S willbe changed to indicate. the same ground radio transmitting stations butwith the spots identitying such stations dashing codes characteristic ofthose stations and also of traiiic conditions in advance for altitude3000. Similarly the contact 24 of the altimeter'contactor selects thenlter portion l2 of the filter FPI, so that the airplane identifyingcode picked up by antenna PA ls one which was radiated lfrom the samealtitude 2000 which was presumably coded by the coder 28 on the sameairplane and radiated from the tail antenna TA of the same airplane butata dierent carrier frequency. and received bythe same or a differentcarrier frequency as passed by receiver SAR, it being received byreceiver PR, and modulated to frequency I2.

As just stated. if desired, the radio information transmitted by groundstations to perform this checking function and picked up by antenna PAon the airplane (Fig. 3) may be transmitted at a different carrierfrequency. In this case an additional ground transmitter and directionalantenna would be employed at each ground station. The antenna PA on eachairplane is of course also of broad angle directional construction forthe particular carrier frequency employed.

As pointed out hereinafter, it is proposed to emit continuous energyproducinga non-flashing spot on the screen under danger, that is, holdtrailic conditions R, to code the radio energy by coding apparatuslocated at the ground station under consideration to ash the radio beamat a high rate for proceed trafc conditions G and at a low rate fornormal traffic conditions N. By normal traffic conditions is meant thetramo conditions that normally exist, namely that there is no airplaneapproaching such fix within two blocks or sections in thelrear for thealtitude in question (3000 foot altitude in the present case). It shouldbe understood that even though a socalled normal radio beam may beemitted'by a ground station this ground station may be potentially clear(N followed by dotted G, see Fig. 1) or may be potentially'hold (Nfollowed by dotted R. see Fig. 1). The filter FPI is. as abovementioned, provided with filter elements FIZ and FIS. The portions ofthe nlter, FSI identified by reference ,characters F2 and F3 signifyaltitudes 2000 and 3000 feet respectively, whereas the tens numeral lof'the characters F12. FI3, and so forth. signifies that only radioenergy shall pass therethrough whichis characteristic of airplaneidentity for these altitudes but coded to identify the airplane on whichit originated and not coded to signify trafilc conditions in advance inthis same altitude. This radio energy is received through the medium ofreceiver PR. and filter FPI. In this connection it should be understoodthat each airplane is provided with a tail transmitter TI which emitsradio energy of carrier frequency F modulated to an altitude frequencyselectedpby the contact 2i and emitted through the medium of arearwardly directed tail antenna TA.

It will be observed that this. tail transmitter IT transmits energy ofcarrier frequency F and of an altitude characterz. 3 or 4 depending uponthe position assumed by the contact 2i of the altitude contact mechanismAC. In other words. when the airplane IP2 is dying at the 2000 footaltitude the contact 2i assumes the position 2 and if the airplane thenadvances to the 3000 foot altitude the contact 2l will be automaticallymoved to select the modulating generator portion F8 of the modulatingfrequency generator FG. The airplane carried apparatus is also providedwith a coder including a coding wheel 28 which codes the radio energyemitted by the tail antenna TA so as to characterize that particularplane.

In order .fon the pilot to conveniently make a continuous check as tothe operativeness of his apparatus, a checking radio receiver CR hasbeen provided. This receiver has an antenna element located in the fieldof the tail antenna TA. This check receiver CR is provided with a filterFCI having associated filter elements F2. F2, etc., which are selectedby the contact 22 of the' altimeter contact mechanism AC. The energypicked up and selected by the particular filter element selected by thealtimeter contact 22 may then flow through the relay W so as to causethis relay to follow the code of the radio beam emitted rearwardly bythe airplane IP2. It will be observed that the filter elements F2 and F8of the filter FPI constitute a second checking instrumentality and areconnected one at a time to the sumed by the contact 2l of the altimetercontactor AC.

From this consideration itis readily understood that the relay W willpick up and drop in accordance with the code transmitted locally by therearwardly directed tail antenna TA whereas the relay Ywill bob up anddown in accordance with the radio energy generated at a ground radiotransmitter, coded in accordance with the coded tail transmitted energy.received at the next ground station lin the rear. These relays W and Yare provided with contacts 21 and.,28 which are included incorrespondence circuits for the relay Z. This relay Z is rather quickacting and by reason of the fact that the two energizing circuits forthis relay Z pass through back contacts and front contacts respectivelyassociated with the movable contacts 21 .and 28 of these relays it isreadily seen that the relay Z will only assume its picked up conditionif the contacts 21 and 28 operate in perfect synchronism. In thisconnection, it should be understood that the contacts 21 and 28 arepreferably make-before-break so that if these contacts 21 and 28 operatein synchronism the .energizatlon of the relay Z is continuous in spiteof code following operation of these contacts 21 and 28. These contacts21 and 28 will of course operate in perfect synchronism if there are noother airplanes around and if the airplane carried apparatus and theapparatus at both the first ground station ahead and the lfirst groundstation in the rear are in proper working condition. j,

The relay Z has associated therewith an indieatingr lamp ZL which willbe lighted only when this relay Z assumes its energized condition.Similarly, the relays W and Y have associated therewith lamps WL and YL,respectively, which for obvious reasons are lighted only when thecorresponding relay assumes its energized position. ,It should also beobserved that the relay W has connected in multiple therewith a lamp 3lwhereas the relay Y hasconnected in multiple therewith a lamp 32. Theselamps Il and I2 will of course follow the code characters impressed onthese relays W and Y respectively. In this connection it should beunderstood that the lamps 3i and 32 may only be used in the event thelamps WL and YL, respectively, are omitted 9 l and that in practice therelay Z and its associated lamp ZL may be omitted il" desired it beingunderstood that the correspondence between the transmitted code and thereceived code may be observed from the lamps 3l and 32 flashing insynchronism. If the relay Z is omitted for this reason thc relays W andY may also be omitted in which event the lamps 3| and 32 alone will berelied upon for the desired check information.

Although many of the devices illustrated in Fig. A3 have been shownrather conventionally it is believed that this conventional showing issufilclentin view of the advanced status of the art in radiocommunication. It may, however, be pointed out that suitable generatorsare associated with the scanning antenna SA for the purpose ofgenerating voltages for application to the sweep plates of the kinescopeincluding the screen B, and these generators have been specifi-f callydisclosed in the applications of Wight et al. and Dicke, above referredto. The letter F assigned to the filters FPI and FSI signify that thecarrier frequency received by the scanning antenna receiver SAR is ofcarrier Vfrequency f whereasthe characters F2, F3, FI2 and FI3, signifyfour different kinds of modulating frequency superimposed on thiscarrier frequency f. As above pointedut two distinct carrier frequenciesmay be used.l It may be pointed out that of these modulating frequenciesthe frequencies F2 and Fl2`have been assigned to altitude 2000, thefrequencies F3 and FI3 have been assigned to altitucle 3000, and thefrequencies F4 and FI4 would have been assigned to altitude 4000 if ithad been illustrated. The modulating frequency generator FG associatedwith the tail transmitterr TT is quency F which is modulated atfrequencies F2 or F3 depending upon the particular altitude 2000 or 3000at which the airplane is flying as determined by altimeter contact 2|(Figs. 3. 5A and 5C). The illter FCI associated with the check receiverCR is of course constructed to |receive carrier frequency F modulated atmodulating frequencies F2 or F3 depending upon the position then assumedby the contact 22 of the altimeter contact mechanism AC. Both of thecarrier frequencies F and f proposed to be employed in the presentsystem are ultra-ultra-short-wave length frequencies in order that theantenna S-A may be highly directional. The tail antenna TA preferablyemits radio energy spread in a conical direction through an angle ofsubstantially 90 whereas the individually focused radio antennae, fourof which have been illustrated, of the scanningantenna SA will onyreceive radio energy within very small spread of the focal axis of theparticular receiving antenna focusing cup then effective. This angle ofspread is preferably about two to five degrees.

Let us now refer'to Figs. 5A and 5B of the drawings andobserve theconstruction of the altimeter contact-mechanism AC shown conventionallyin Fig. 3 ofthe drawings and illustrated more specifically in Figs. 5A,5B and 5C.

Referring to Fig. 5A the altimeter contact mechanism includes a bellowsor Sylphon SY which is evacuated and which has a collapsing tende-ncyagainst the force of the spring 35 dei pending on the atmosphericpressure and therefore depending on the altitude at which the airplaneis flying. This Sylphon SY is held in a predetermined expanded conditiondepending upon the atmospheric pressure by a U-shaped spring 35 and isprovided with an arm 36 which will sweep about the U-shaped part of thespring 35 as a pivot through an arc dependent upon the change inatmospheric pressure andwill thereby, through the medium of the arm 36and the link 31, rock the angle lever 38 supported for movement about a.pin 33 so as to cause the arm 38 to pull or release the chain 4iagainstthetension of the .hairjspring 3l)L depending upon whether theatmospheric pressure is rising or falling respectively. As a result ofthis action the pointer 45 is operated through the medium of the drum 44in a clockwise direction if the atmospheric pressure is falling(altitude increasing) and in a counter-clockwise direction when theatmospheric pressure is increasing (altitude decreasing). In otherAwords, the altitude indicating pointer 45 is moved in a clockwisedirection as the altitude of the airplane is increased.

In order to have the pointer 45 indicate the altitude accurately inspite of variations of the atmospheric pressure at sea level themanually operable adjusting knob AK is provided to adjust the positionof t e'v dial plate 41. 'I'his dial plate 41 may be adius by the pilotin accordance with information received by hlm through the medium ofradio communicationrand may be adiusted to a position to indicate withrespect to a stationary pointer 34 the atmospheric pressure at sea levelfor that particular locality. This turning of the manually operable knobAK, through the medium of the pinion 46, will rotate the toothed scaleplate 41 with respect to which plate the pointer 45 is read. .In otherwords. if the knob AK is adjusted to indicate atmospheric pressure atsea level by pointer 34 the pointer 45 will fndicate the altitude atwhich the airplane is nying under this atmospheric pressure condition.Itis this latter construction that distinguishes the altimeter from abarometer. At the end of the shaft 43, on which the chain drum 44 andpointer 45 are supported, is provided a mirror MI for-purposes presentlyto be described.

A suitable light source such as a lamp L is provided on the axis of theshaft 43 beyond the mirror MI. Between this lamp L and the mirror l MIis provided a condensing lens CL which concentrates the light emitted bythe lamp L into a narrow beam striking the mirror MI at the Ipointsubstantially in the axis of the shaft 43. It is readily seen that bythis construction themli'zht beam reflected by the mirror MI will rotatein n plane at right angles to the axis of this shaft 43 and that thislight beam will rotate in exact synchronism with rotation of the shaft43. Adjacent this rotating light beam is provided a wheel rim 48 whichsupports two light responsive cells LCI and LC2. These light responsivecells are included in the circuits of relays/DR and UR respectively.These cells may be of selenium construction but are preferably lightresponsive Ycells of the photo-electric type. The cell LCI is showncurrent produced by the photo-electric cell"".\ A -similar circuit forthe relay UR may be traced from the terminal (-i-), slip ringj, throughthe light responsive cell L02, winding of the relay UR, and back contact50 of the relay DR. 'Ihese relays DR and UR are provided withcontacts-5|, 52, 53 and 54 which for obvious reasons will cause currento one polarity or theother to be applied A to the mptor M of thepermanenbmagnet field type or induction type, the polarity dependingupon which of the two relays DR or UR assumes its energized condition.

It will be observed that the relay DR may be picked up by depression ofthe push button contact PD whereas the relay UR may be picked up bydepression of the push button contact PU. These push button contacts areprovided for test purposes and should not be used during route flying.It will be observed (see Fig. A) that the motor M (also shown in Fig.5B) is operatively connected to the gear 42 rigid with ring 48 throughthe medium of the gear 55. It is thus readily seen that if the pointer45 and the light beam are moved clockwise by the altimeter from itsnormal position in which the light beam strikes between the two lightresponsive cells this light beam will implnge on the light responsivecell LC2 thereby causing the flow of current through the relay UR whichresults in the operation of the motor M in a direction to causeclockwise rotation of the rim 48 (as viewed from the right in Fig. 5A)to cause the two light responsive cells LCI and LC2 to again assumepositions on opposite sides of the light beam in its new location. Inother words, the light responsive cells LCI and LC2 are caused to assumepositions corresponding to the position assumed by the altitude pointerB6. p

In practice the dial 4l back of pointer 45 is provided with indicationsindicating only altitudes up to an altitude of 10,000 feet whereas inpractice airplanes will ily in altitudes up to 20,000 feet.

In other words, the pilot must be relied upon to' properly read andinterpret the altimeter including the pointer t5. In order to providecontacts which complete a different circuit for each altitude between1,000 feet and 20,000 feet contact arms 20, 2E, 22 and 20, of which thecontact arm 2l only has been shown, are provided (see Fig. 3). TheseContact mechanisms are driven through gear mechanism so as to rotateonly one complete revolution for two complete revolutions of the pointerThis gear reduction is provided through the medium of the gears (l2 and5l, the gear lill having twice the diameter oi' gear 32.

12 therewith has been indicated by the letters WDAA. This directionalantenna WDAA directs its radiating energy upwardly and rearwardly withrespect to the direction of 4travel over the route, the route underconsideration being an eastbound route.` This radio transmitter WRAtransmits radio energy of ultraultra short-wave length of frequency fand has associated therewith other means for modulating this carrierfrequency f at modulating frequencies, two frequencies for each altitudeof airplane flight. As shown for fix A this transmitter WTA hasassociated therewith a frequency generator FG capable of Vmodulating thefrequency f at modulating frequencies F2 and FIZ for altitude 2000, F3and FI! for altitude 3000,

' and F4 and FH (not shown) for altitude 4000.

The contact arms 20, 22 and 20 are also carried by the shaft 58 whichcarries the contact arrn l. Of these contact arms E0, 2l, 22 and 20 thearrn ai is more specically shown in Figs. 5A and 5B of the drawings. Inorder to provide an additional indicator which does not requireinterpretation by the pilot as to whether he is ying in the rst i0,000feet of altitudes or the second 10,000 feet of altitudes a secondpointer 00 (see Figs. 5A and 5B), which rotates only half' as fast asshaft e3, has been provided to visually indicate the actual altitude atwhich the airplane is flying.

Ground located structures-ln Figs. 4A, 4B and 4C have been shown theground located apparatus, for three fixes at the approach to an alrroutecrossing illustrated in Figs. 'iDXi and 4D'X2. The expression DXrepresents a' iii: D of an eastbound route combined with a fix X of anorthbound route crossing the eastbound route at fix D.

The apparatus at each fix includes a radio transmitter T having a prexletter corresponding to the direction in which a radio directionalantenna is directed and a sufllx`fcorresponding to the letteridentifying that iix. For fix A (Fig.

.c 4A) this radio transmitter has been designated WTA and thedirectional antenna associated Similarly, each fix, such as fix A (Fig.4A) is also provided with a radio receiver ERA, the letter E designatingthat the directional antenna EDAA associated therewith is directedeastwardly to receive radiated energy from the tail end of a departingeastbound airplane. This receiver ERA has associated therewith a filterFI which is capable of receiving ultra-high carrier fre-v quency Fmodulated at either the modulating frequency. F2 or F3, as conditionsrequire depending on the altitude at which such airplane is flying. Infact, radio energy may be received modulated to a plurality of thesefrequencies, each modulating frequency being used to perform aparticular function associated with the altitude to which suchmodulating frequency is assigned. It is of course understood that thesemodulating frequencies F2, F3 are associated with altitudes 2000 and3000, respectively.

For each altitude at each x there are provided a group of relays ALRP,AR, ARP, TR. TRP, TRE?, TLR, TLRPF, TLRPB, CR, CRP and TE. The referencecharacters in the speciflcation for each of these relays has a properprex to identify the x and the altitude with which that particular relayis associated. For instance, these relays for fix A altitude 2000 areeach provided with 'a prefix A2. and similarly, these relays for thealtitude 3000 for x B are provided with a preix B3.

It is readily seen that the relay TR for altitude 2000, fix A, isconnected to the output side of the filter FT associated with the radioreceiver ER in a manner to receive only radio energy of carrierfrequency F and modulated at frequency F2. In other words, this relay TRwill respond to energy radiated from the rear of an airplane departingfrom iix A in an eastwardly direction provided that this airplane ies ata 2000 foot altitude and radiates energy of carrier frequency Fmodulated at modulating frequency F2. Since each airplane (Fig. 3)radiates rearwardly directed energy characteristic o1 the altitude atwhich the airplane is flying and coded at a rate to characterize theparticular airplane, the tail relay A-TR (Fig. 4A) will follow the codeof an airplane flying eastwardly at an altitude 2000 and departing fromfix A, which energy is coded to characterize that airplane. Therefore,the relay i2-TR may be conveniently called the tail relay for fix A,altitude 2. This relay A-TR (Fig. 4A) has associated therewith aslow-dropping repeater tail relay AZ--TRP which remains up for any codeand which in turn controls a second repeater relay A2-TRPP.

Referring now to Fig. 4B it will be observed that the tail relay B2-TRthrough the medium of its contact |63 repeats the code received there-13 by lnto the tail line repeater relay AR-TLR. (Fig. 4A). In otherwords, if an eastwardly flying airplane flying at the 2000 foot altitudein block B-C radiates a code characteristic of that airplane this radioenergy code will be received by both the directional receiving antennaEDAA (Fig. 4A) and the eastwardly directional radio antenna EDAB (Fig.4B) to cause codeI following operation of both of the relays .A2-TR andA2 TLR (Fig. 4A) in unison or synchronism. It should be observed thatthe tail line relay A2- TLR (Fig. 4A) is provided with a front contactrepeater relay A2-TLRPF and a back contact repeater relay A2--TLRPB,both of these relays A2-TLRPF and AZ-TLRPB are slow dropping or decodingrelays which remain up continuously so long as the relay A2-TLR followsany airplane code. Code following operation of the relay A2-TLR willtherefore cause both of these repeater relays A2-TLRPF and A2'I'l..RP.`Bto assume their energized positions continuously.

Since the circuit for the correspondence relay A2-CR includes a frontcontact |36 of the relay A2-TLRPB and a front contact |39 of the relayAZ-TRP as well as correspondence contacts |31 and |38 of relays A2-TRand A2TLR respectively, for both simultaneous energlzation andsimultaneous deenergization of these relays, it is readily seen thatunder the condition assumed, namely. with an eastbound airplane ying at2000 foot altitude at a point just beyond fix B will cause thecorrespondence relay AZ-CR to be substantially continuously energized.VVThis relay A2-CR is a slow pick-up relay, requiring say about tenseconds to pick up, and since itis also quick droppingvit will drop inresponse to a mo-l mentary deenergization of its energizingv circuit andwill only be picked up if synchronous operation of relays A2-TR andA2-TLR takes plac( If desired the contacts |38 of relay A2--TR and^` thecontacts |31 of -relay A2-TLR may be con'- structed make-before-break sothat the circuit for the correspondence relay A2-CR is not at all brokenduring synchronous code following operation of the relays A2--TRfandA2-TLR when following the same code. It should be understood that thetail repeater relay A2-TRP is also a decoding relay slow enough indropping so as to remain up continuously during the reception l of anyairplane code by the tail`relay A2-TR.

The correspondence relay repeater A2-CRP is preferably slow dropping toa slightly greater extent than the repeater relay A2-TRP is slowdropping. This timing is resorted to so that dropping of the relayA2--TR, as by an airplane getting out of range, will not .drop timeelement relay A2-TE. The airplane getting out of range willcauserelayA-TRP to drop -causingmomentary opening of contact |64 of relay A2- TRPPwhen contact M5 of relay A2-TRP is ope'n and this function must takeplace while front contact |14 of relay A2-CRP ls still closed to preventdropping of time element relay A2.-TE,

As an exampleif the repeater relay AZ-TRP requires say one second todrop it is proposed to design its associated relay Ail-CRP so asto re--quire' about 11/2 seconds to drop. The back contact |12 of relay AZ-CRof course closes before front contact |13 of relay A2--TCRPopens andtime element' relay A2TE is 1 held energized through contacts |12 and|14 of relays AZ-CR and A2-CRP while contacts lfand ISB of relays A2-TRPand A-TRPP are both. open.

Also, the relay A2-TCRP is slightly slower to rey lease than the sum ofthe pick-up times for reassess:

' 14 lays A2-CR and A2-CRP. This provision oi' timing is desirableand\is resorted to in order to prevent deenergiz'ation of the timeelement dew vice A2-TE when an airplane passes the nextl 5 location inadvance resulting in the picking up of relays A2-TLRPF, .A2-CR andAZ-CRP; The picking up of relay A2-TLRPF by the opening of its backcontact I causes deenergization of relay A2--TCRP (this will however notdrop by l0 reason of its slow dropping feature) and by the synchronousoperation of contact |31 of relay A-TLR with contact |38 of relay .A2-TR will cause picking up of relays A2-CR. and A2-CRP, and thetiming'just mentioned will cause closing of front contact |42 of relayA2-CRP before the drop-away time of relay Alk-TCR?. has expired, so thatwith the` relay 4.li/l-TCRP, now energized through front contact |42 ofrelay A2--CRP the relay A2-TCRP, ywill not drop at allg/It may bepointed out here that these timing features are also required to causedropping of the timeelement relay A2-TE when\'l an airplane circulatesto perform a holding function. That is, when an\airplane at 2000 feet in2;, section A-B, and in the/rear of another airplane in section B-C,turnsnough to get out of tail energy transmitting'relationship with theground receivers t station A the relays A2-TR land A2-TRP wil begin topick up and drop in synchronism which results in the picking up of relayA2`CR`-opening of its back contact |12, and dropping of time elementrelay AZ-TE, the relay A2-TCRP under the assumed condition being in itsdeenergized condition.

These specific delay times may be varied dependingupon the codesemployed and the other factors/ and the particular pick-up time of tensecon sor relay A2-CR and dropping time of say tw ive seconds for relayA2-TCRP assumed 4o are merelyexemplary for the purpose of disclosure ofthe present invention, it being understood that the applicants do notlimit themselves to these speciilcy delay times. y

The time element relay TE is very quick drop- 45 ping and is so slow inpicking up that it requires substantially five minutes of continuousenergization to pick it up. Obviously an ordinary relay could not beused for this purpose and it is proposed to employ a motor operated timeele- 5" ment device, such, for instance, as disclosed in Fig. 5 of thepending application. of Field, Ser. No. 476,207, filed February 1'7,1943n now Patent No. 2,378293 dated June l2, 1945. When this timeelement device of the Field application is employed the shunt-outfeature performed by relays AR and DTR disclosed therein will of coursebe omitted, this omission of the shuntout feature is equivalent tohaving the contactsL I6| and IIB of relays AR and DTR' continuously 7"open. If this time element device of the Field application is employedherein the wire leading from contact ||`d of relay MR (shunting contactsHs and na of relays AR and DTR omitted) of such prior applicationbecomes the wire leading into the winding of the relay TE of the presentinvention and the contacts |54 and |68 of the relay TE in the instantapplication will be controlled by the relay LR of the prior application.In other words` the motor timing apparatus and associated relays of theprior Field application may be substituted for the relay TE of theinstant application. It is readily understood from the prior applicationof Field that momentary opening of the contact il# of the Fieldapplication results in immediate opening eficaces s period has beenconsumed. Obviously, the relay CR. for the various altitudes and fixesof the instant application may also be replaced by a time elementstructure such as disclosed in the prior Field application. K

It may be pointed out here that the control circuits for the variousrelays for the various altitudes for fixes A and B (Figs. 4A and 4B) areidentical and that corresponding apparatuses for the fix C are onlyslightly diierent by reason of the fact that this fix C is an approach xto an automatic crossing fix DX shown in Figs. 4DX1 and 4DX2 of thedrawings. One of the principal differences is that the tail linerepeater relay C2-TLR is a normally energized relay whereas thecorresponding relays for fixes A and B are normally deenergized relays.For this reason it is necessary that the correspondence contacts |36 and|31 for relays 'C2-TR and CZ-TLR. be inversely connected to that of theconnection, of similar contacts for the relays at ilxes A and B. Thatis, in Fig. 4B the correspondence relay B2-CR will only pick up if therelays BZ--TR and B2-TLR pick up and drop in synchronism whereas thecorrespondence relay (J2-CR, (Fig. lfiC) will only pick up if the relay(J2-TR is in an energized condition each time the relay C2@- TLR is inav deenergized and vice versa. There is another slight dierence betweenthe struc tures shown in Figs. 4B and 4C in that the relay CZ--TLRPF isprovided with an additional con tact IE5 not provided for the relayBtw'ILRiPlt".

The purpose of this additional contact is to pro1 vide two hold signalsfor airplanes approaching an automatic crossing. lhat is, if thecrossing is not available this is manifested not only at the crossing xbut is also manifested' at the approach fix.

At each nx there is provided suitable coding apparatus which for thepurpose of disclosure f of the invention only has been illustrated bycod lng wheels APW, ANW and ARW for rie.; (Fla. 4A) These code wheelsare driven by e, coding motor AM which has been conventionally illusutrated as an induction motor. En practice there is preferably a gearreduction between the motor AM and the code wheels APW, ANW and ARWwhich gear train for convenience has been rannteV ted. Similar codewheels EFW, PNW and lt?,

DXRW driven by motore BM, CM and DXM.', re

, spectively, have been illustrated for fixes B, C and DX, respectively.

is illustrated, a portion of each code wheel transmits a codeidentifying the fix whereas the .remaining portion ci the code wheel isconm lar teeth li which are wider and spaced cornea., what farther apartwhich teeth are provided tol produce a low ram or normal code. Similarteeth l@ and 1| are provided for corresponding code wheels illustratedin Figs. eB, eC and @DX of the drawings.

Reerrine if@ Fie. te. the code wheels fri' ANW and ARW are provided withteeth 12 and li lor characterizing the letter A, the tooth 12 being adot creating tooth and the tooth 13 be ing a dash creating'tooth. Thecode wheels BPR, BNW and BHW on the other hand are provided with teeth1li, l5, 16 and 11'to characterize the letter B which identifies iix B.

Referring :now to Fig. 4C the teeth 18, 19, 80 and 6| on the code wheelsCPW, CNW and CRW are of widths and are arranged in succession tocharacterize the letter C.

Referring now to Fig. 4DX the code wheels DMW, DXN'W and DXRW areprovidedv with coding teeth 82, 83 and 84 to signify the letter D, andteeth 85, 88, 81 and 8B to characterize the letter X. These code wheelsDXPW, DXNW and DXRW are driven by a coding motor DXM.

Directly over these code wheels for ilx A is provided a three-wire busconsisting of bus wires 9U, 8| and 92. It is readily seen the bus Wire8l during the.trailc controlling portion of rotation oi f the codewheels receives uncoded energy. through code wheel ARW from theassociated battery ABI (ilx A), the batteries at other xes beingdesignated BI with a. preilx corresponding with the x at which it islocated. The bus wire 9i' is coded by the coding wheel ANW (Fig. 4A) andthe bus wire 9.2 is coded at a high rate by the code wheel APW (Fig.4A.) Although any form of commun; :ation between xes may be resorted to,such as r dio communication, for convenience line Wires ed, et, 98, 9s;C, ll, |02 and CC have been provided between fixes A and B. The wire *Chas included in series therein a battery ABZ whereas the wire CC hasincluded therein a battery Ant. The section between fixes B and C isprovided with similar line wires M5, |06, 08, we, C, lil, M2 and CC ofwhich the line wire C has included therein a battery B32 and the linewire CC has included therein a battery BB3. Similar line wires areprovided between fixes C and DX except that each wire is indicated by anumber ten units greater, and of which the wire C has included in seriestherein a battery CB2 and the Wire CC has included in series therein abattery GBS.

Referring now to altitude 2600, nx A (Fig. 4A), it will be observed thatthe tail relay AZ-TR is energized through a circuit including the Wiresi3@ and 93E which. connect this relay A2-TI-l. to the radio receiver ERAthrough the medium of the filter section F2 of the lter FI. in asirnilar manner the relay rid- TR is connected to this receiver im.through the medium of wires itl and 932. 'it will be observed that relayi2- P is controlled through a circuit including the :iront contact it@of the relay TR and that the re peater relays Ali-TLRPF and A2TLRPB arecontrolled through iront and back contacts re speotively including themovable contact it ci the relay l2-JT. The correspondence relay f CR lecontrolled through a circuit including front contact i3d of the relayl2-TLRPB correspondence contacts lill and |32 o the relays AZ--TLR andMd of the relays A2-CRP, .A2-TRP and Ali-TLRPF respectively. inmultiple.

The time element `device AZ--TE is controlled by a control circuitincluding in series two groups of contacts each group of which includesa piu- Maasai raiity of contacts in multiple. The nrst of these groupsof contacts include back contact |12 o! relay A2CR andV front contact|13 of relay A2- TCRP, and the second groups of contacts include frontcontacts |15, |45 and |64 ot the relays AZ-CRP, AZ-TRP and AZ-TRPP,respectively, in multiple. It should be observed that the tail repeaterrelay AZ-TRPP is controlled through both front contacts and backcontacts each including the movable contact |46 oi' the relay A2- TRP sothat this relay AZ-TRPP is energized irrespective of whether the relayA2TRP assumes its energized or its deenergized position. It may,however, be pointed out that the windings of this relay AZ-TRPP areoppositely'poled so that each time therelay A2TRP operates from anenergized to a deenergized position or from a deenergized to anenergized position the relay AZ-TRPP assumes its deenergized positionmomentarily. This is by reason ot the fact that pole changing oi' themagnetism in the core oi' this relay AZ-'I'RPP requires the magnetismduring its pole changing operation to go through zero value and thiscauses this relay to momentari'y assume its deenerglzed position.

Referring now to ilx (Fig. 4B) it should be observed that the approachrelay B2-AR is controlled through a circuit which may be traced from theterminal of the battery AB2, wire 89, back contact |41 of the relayA2-CR, front contact |48 of the relay A2-TR, wire 68, winding of theapproach relay B2AR, and wire C, back to the terminal of the batteryAB2. Similarly, the approach repeater relay BZ-ALRP is controlledthrough 'a circuit beginning at the (-l-) terminal of the battery AB2,wire 69, front contact |49 of the relay AZ-ARP (Fig. 4A), wire 99,winding of the relay BZ-ALRP, and wire C, back to the terminal of thebattery ABZ. The purpose ot back contact |90 oirelay TE will be pointedout hereinafter. It is thus seen that the approach relay B2-AR directlyrepeats the tail relay A2-TR for the same altitude at the next ilx inthe rear providing that the correspondence relay .CR at such iix .in therear assumes its deenergized position. It is also observed that theapproach repeater relay BZ-ALRP directly repeats the approach repeaterrelay ARP for the next x in the rear through the mediumY of the irontcontacts |49 of such approach repeater relay in the rear. In thisconnection it should be observed that the approach repeater relayAil-'APR is a slow dropping relay and it should be understood that thisrelay will remain up continuously if its associated approach relay A2-ARis energized in code fashion. In other words, the relay AZ-ARP issuiliciently slow dropping'so as to remain in its 'energized positionbetween code impulses.

It should also be observed (see Fig. 4A) that the wire |50 whenVenergized is capable of causing the emission of radio energy of carrierfrequency f and modulated at frequency F2. It will then be seen thatthis wire |50 is provided with uncoded energy signifying stop or holdingtrame conditions in advance and, for altitude 2 fix A, this wire |50 isso energized'through a circuit starting from the (-1-) terminal of thebattery ABI, wire 65, brush 61, slip ring 68, code wheel ARW, brush 69,bus wire 90, back contact |5| of the relay A2-CRP, i'ront contact |52 ofthe relay A2-TRP, and front contact |53 or |55 of the relays AZ-ARP andAZ-ALRP. It is readily seen that this uncoded energy is changed toenergy coded at the high impulse rate (code wheel APW) if either thecontact 352 oi the relay .A2-TRP assumes its deenergized position withthe contact |54 oi relay A2-TE remaining in its energized position, orboth if the contacts |5| oi.' relay A2-CRP and |54 of relay A2--TEassume their energized position with front contact |52 still closed. Itis also readily seen that front contact |55 oi' relay AZ-ALRP is inmultiple with the front contact |55 of the relay .A2-ARP so that witheither one of these approach relays assuming their energized positionthe same partial circuit is closed. Furthermore, it is readily seen thatif both of the contacts |55 and |55 ot the relays AZ-ALRP and .A2-ARP.respectively, assume their deenergized position, that the transmittingfeed wire |50 becomes energized by current pulsed at the low impulserate to cause the radio transmitter WTA to emit the normal tramoindications.

As already pointed out hereinbefore, an airplane as it illes along theroute emits so-called tail radio energy rearwardly which is received byone or more radio ground antennae in the rear and that this radio energyis retransmitted from the ground back to the same airplane through themedium of a diier'ent radio carrier frequency f transmitted from anantenna at the next fix in advance and impulsed at a code characteristicof that same airplane. As already pointed out the relay B2--AR directlyrepeats the tail relay A2TR and attention is now directed to the factthat the front contact |56 of the approach relay BZ- AR is the contactwhich repeats the code received by the tail relay A2-TR into the 'radiotransmitter WTB at tix B through the medium of the carrier frequency fgenerated by frequency generator FG through a circuit including the wire|59. The current flowing over wire |50 is coded in accordance withtraillc conditions and is modulated at frequency F2, and the currentowing over wire |56 is coded to identify the airplane directly in therear of that iix and is modulated at frequency FI2. It is thus seen thatwires |50 and |59 supply energy to the radio transmitter WTB forgenerating two kinds of carrier frequency current for altitude 2, and ina similar manner wires |6| and |62 supply energy for similar purposesfor altitude 3000 feet. Supplemental approach control in the form of aback contact onrelay TE is provided so that occupancy of a block wherecircling is taking place is manifested in advance.

The various code wheels, relays, contacts and wires for ilx A altitude 2have been identified by certain reference characters. It is now desiredto point out that like wires and contacts at other fixes and altitudeshave been assigned like reference characters except for the hundredsdigit, whereas like relays, code wheels, transmitters. receivers',antennae, and the like, at the various iixes have been assigned likereference characters except for a prefix identifying the particular fixand the particular altitude of such fix involved.

The apparatuses for the fix portion D and the 'fix portion X foraltitude 2000 for ilx DX illustrated in Figs. 4DX1 and 4DX2 areidentical to the apparatus shown for fix B or C except for the provisionof controls imposed by two interlocking relays WElR and SN'IR, where theletters W, E, S and N denote directions. It is readily seen that theseinterlocking relays WEIR and SNIR have their pick-up circuitsinterlocked each through the medium of a back contact of the otherinterlocking relay.

accesar Referring to Fig. 4DX2 it will be observed that the interlockingrelay SNIR may be picked up through a pick-up circuit starting at thetereither the front contact |69 of the relay D2.

ALRP or the front contact of the approach relay DZ-ARP. Similarly,interlocking relay WEIR is provided with a pick-up circuit which may betraced from the terminal 'of a suitable source of current, back contactof the relay X2-ALRP, back contact |16 of the relay )I2-ARP, backcontact |11 of the interlocking relay BNIB, wire ISI, and the winding ofthe interlocking relay WEIR. Also, that interlocking relay SN'IRincludes a stick circuit including its stick contact |18 and includingin multiple front contacts |19 and |80 of the relays X2 ALRP and)I2-ARP, respectively.

Another`difierence between the structures at an interlocking fix and atan ordinary ilx is that the contact |63 of the tail relay TR at an ordi-20 wardly and downwardly as shown in Figs. 3 and 4C so as to activatethe eastwardly directed directional antenna EDAC and EDAB (Fig, 4B).This radio emitted tail energy will of course be coded to characterizethe airplane |P2 as already described in connection with Fig. 3 of thedrawings. Furthermore, this radiated radio energy will be of a frequencyF uponewhich is superimposed a modulating frequency F2 so that the radioenergy received by both of the radio receivers ERB (Fig. 4B) and ERC(Fig..4C) will freely flow through the filter portion F2, from whencethe coded current may flow down the wire vat each ilxthrough the.winding of the relays nary fix is a front contact whereas the cortheinterlocking fix next in advance thereofwhereas the tail line relay atan ordinary fix such as relay A2--TLR (Fig. 4A) is deenergized underfavorable tramo conditions in advance in that it is controlled through afront contact |63 of the tail relay B2YTR for the same altitude of thenext fix B in advance (Fig. 4B). The contact |84 (or 364) of aninterlocking relay, when open. causes occupancy manifestation at the fixin the rear by dropping of the tail line relay TLR. Also, since contact|85 has been shifted from relay CZ-TLRPF (Fig. 4C) to constitute a backcontact |85 of the'interlocking relay WEIR (Fig. 4DX1), this lattercontact causes a danger aspect to be displayed at the interlocking fix.

Operation Operation over affronta-By referring to Fig. l it will beobserved that in altitude 3000 there has been illustrated only oneairplane |P3 flying eastwardly and approaching the fix A whereas for the2000 foot altitude three eastbound airplanes IP2, 2PZ and 3PZ have beenillustrated approaching the fixes D, B and A. respectively.

These airplanes |P3, |P2, 2PZ and 3PZ have alsoV been illustrated inFigs. 4A-4C and additional airplanes have been shown in Fig.y 1 of the.

BZ-TR and C2-TR and then back up through the common wire |3| connectedto the filter FI of which there is one associated with each of the tworadio receivers ERB and ERC. The received current after being detected(rectified) will not be able to flow through any other filter. such asfi'ter F3. Obviously, the two tail relays BZ-TR and CZ--CR will operatein synchronism to characterize the code emitted by the airplane carriedapparatus of airplane (P2. At fix C (Fig. 4C) the code followingoperation of the tail relay C2-TR and the intermittent closing of itscontact |34 will cause the tail repeater relay CZ-TRP to assume itsenergized condition and remain continuously energized. With this relayCZ--TRP assuming its energized condition, resuting in the closure of itsfront contact |52, it will cause the radio transmitter WTC (Fig. 4C) toemit radio energy conventionally illustrated by the dotted line R2 tothe airplane 2PZ flying in the second section to the rear of the sectionin which the airplane |P2 is flying. This emitted radio energy will beof carrier frequency l and will be modulated to moduating frequency F2(meaning the modulating frequency manifesting trafiic' conditions foraltitude 2000) This radio frequency energy will be uncoded therebysignifying danger traffic conditions in advance meaning that an airplaneflying in block B-C and receiving such indication must hold and not passthe fix C. The circuit for suppying this uncoded energy to the westradio transmitter WTC may be traced from the terminal of the battery CBI(Fig. 4C), wire 60, brush 61, slip ring 03, code wheel CRW, brush B9,wire 00, back contact |5| of the relay CZ--CRP, front contact |52 of therelay C2-TRP, front contact |05 of the relay CZ-TLRPF, front contact |55of the lapproach re'ay C2-ALRP, wire |50, theportion F2 of themodulating frequency generator FG, wire |69, back to the other terminalof the battery CB. This front contact |55 would not be closed were itnot for the fact that thereis an airplane approaching x C within twoblocks in the rear. The pilot in the airplane 2P2 (Fig. 4A) is thusinformed of danger tramo conditions in the second block in advance ofhis airplane. As already pointed out this danger indication (R2) is inVpart due to the fact that his own airplane 2P2 radiates tail energy tocause the approach relay CZ-ALRP to assume its energized position toclose its front contact |55 included in the circuit lust traced. Thereason for this approach relay C2--ALRP assuming its energized positionwill be described presently.

Code following operation of the tail relay CZ-TR will through the mediumof its front contact |63 cause code following operation of the tail linerelay B2-TLR (Fig. 4B) through a circuit which may be traced from theterminal of the battery BB3, wire |34, winding of the relay BZ-TLR, wireH2, front contact |63 of 21 the relay C2TR, through the common returnwire CC connected to the other terminal of the battery BBS. For likereasons as above pointed out in connection with altitude 2000 at fix C,the tail relay B2-TR .will be picked up intermittently and the repeaterrelay BZ-TRP (Fig. 4B) wil also asume its energized positioncontinuously in response to the reception of radio energy by thereceiver ERB and from airplane |P2. It is thus readily seen that the twotail relays B2-TR and B2-TLR will pick up and drop in synchronism, theformer receiving its energy through the medium of receiver ERB and thelatter through the medium of receiver ERC. as a result of which thefront contacts |31 and |38 of relays B2-TLR 'and B-TR will pick up anddrop in synchronism. Also, the intermittent dropping of the tail linerelay B-TLR will through the medium of its back contact |35 causeintermittent energization of the back repeater relay B2-TLRPB whichresults in this latter relay assuming its energized positioncontinuously. The following circuit for the correspondence relay B2-CRwill therefore be substantially continuously c'osed: starting at theterminal of a suitable source of current, front contact |36 oi' therelay B2-TLRPB, contact |31 of the relay B2-TLR, contact |38 of relayB2-TR,V front contact |39 of the relay BZ-TRP, through the winding ofthe correspondence relay B2--CR, to

' the other terminal of said source of current. The

relay B2-CR is very quick dropping and very slow picking up so that itis assured that this relay BZ-CR will only asume its energized positionii the relays B2-TR and B2TLR operate in perfect synchronism. Thepicking up of the contact of the correspondence re'ay B2-CRP, this relaybeing a slow dropping repeater relay for the 'relay B2-CR, causesproceed coded energy, coded at a high rate' by the code wheel BPW, to beapplied to the west transmitter WTB instead of non-coded energy as wasthe case at this 2000 foot altitude of iix C with the airplane |P2 stilloccupying the section C-D.

The circuit for transmitting this proceed coded energy may be tracedfrom the battery BBI, wire 56, brush 61, slipring 68, code wheel BPW(Fig. 4B), contact brush 65, bus wire 92, front contact |54 of the timeelement relay B2-TE, front contact |5| of the correspondence repeaterrelay B2-CRP, front contact |52 of the relay B2-TRP, front contacts |53and |55 of the relays B2-ARP and B2ALRP, in multiple, wire |50, to theportion F2 of the modulating frequency generator FG through the commonreturn wire |69 to the other terminal of the battery BBI. The dottedline G2, extending from the radio transmitter WTB at nx B to theairplane 2P2, signifies the reception of proceed traflic condition radioenergy by the airplane 2PZ insofar as the ground station image in thelower portion of the screen S of his kinescope is concerned. In theupper portion of the the front contact |54 of the relay B2-TE ineludedapproach relay contacts |53 and |55.

We will now point out how the code following approach relay BZ-AR (Fig.4B) and its slow dropping repeater relay B2-AP are picked up at ilx B(Fig. 4B) and how the approach relay 22 C2-ALRP at nx C (Fig. 4C) iscaused to assume its energized position continuously 'under the trafficconditions illustrated in Figs. iA-4C. Reierring to Fig. 4A it will beobserved that, the radio tail energy emitted by airplane 2PZ strikes thedirectional antenna EDAA to thereby cause the tail relay AZ-TR to followthe code characteristic of the airplane 2P2. Code following operation ofthe relay A2-TR through the medium of its contact |49, with back contact|41 of the relay A2CR closed, will cause code i'ollowing operation ofthe approach relay BZ-AR (Fig. 4B). The circuit for causing suchoperation oi the latter relay may be traced from the (+7) terminal ofthe battery AB2, wire 69, baci: contact |41 of the relay AZ-CR. frontcontact |43 of the relay A2-TR, wire 96, ,winding of the relay BZ-ARthrough common return wire C, and

-back to the other terminal of the battery ABZ.

The circuit just traced will cause code following operation of the relayB2-AR which through the medium of front contact |51 of this relay willcause the approach repeater relay BZ-ARP to assume its energizedposition continuously. With this latter relay BZ-ARP continuouslyenergized it will through the medium of its front contact |49 cause theapproach line relay C2- Y ALRP to assume its energized position contin--lli . BPW.

uomly through a circuit which may 'be traced from the terminal (-1-) ofthe battery BB2, wire 89, front contact |49 of the relay BZ-ARP. wire|09, winding of the relay C2--ALRP, to the common return wire Cconnected to the terminal of the battery BB2. It is thus seen that eachairplane will produce approach control not only at the iirst radio x inadvance of the airplane but also at the second fix in advance oi theairplane. It is readily seen that if both of the approach relays, suchas BZ-ALRP and B2--ARP assume their deenergized position that thatparticular altitude for the fix under consideration will display anormal coded signal for airplanes approaching from the rear at otheraltitudes. This is readily understood from the fact that back contacts|55 and |56 of these rclays B2-ALRP and BZ-ARP will apply energy to theradio transmitter WTB coded at the normal rate accomplished by the codewheel BNW. It will be observed that the coding teeth 1| of the codewheel BNW are wider and farther apart than are the coding teeth 10 ofthe code wheel In other words, the code wheel BPW transmits a high orproceed code, whereas the code wheel BNW transmits a low" or normalcode. It should also be remembered that the trailic condition code isintermittently interrupted long'enough to transmit the fix identifyingcode.

This feature of transmitting a low rate or normal code is indicated forairplane |P3 (Fig. 4A) in that for this airplane, by reason of havingalmost approached lx A, the pilot thereof can observe the indicationsemitted by three radio fixes in advance. In other Words, this airplane|P3 will observe a proceed signal at fix A as indicated by the dottedline G3 (Fig. 4A), will observe a second green signal'as indicated byanother dotted line G3 which extends from the radio antenna WDAB at fixB to the airplane |P3 and will observe a normal indication at x C asconventlonally illustrated by the dotted line N3. All of these dottedlines G3 and N3 (Fig. 4A) signify radio beams transmitted from a radiotransmitting ground located antenna to a radio receiving

